1. Field of the Invention
The invention relates to a technique for detecting an abnormality in protection circuits each including an overvoltage protection element provided in correspondence with a corresponding one of a plurality of electrically serially connected electrical storage elements.
2. Description of Related Art
In order to avoid a reverse charge state of each single cell, a Zener diode may be electrically connected in parallel with each of a plurality of electrically serially connected single cells. For example, when any one of the single cells enters a reverse charge state, discharging current is caused to flow to a corresponding one of the Zener diodes and to bypass the any one of the single cells. Thus, it is possible to cancel the reverse charge state of the any one of the single cells.
However, when any one of the Zener diodes fails and there occurs an abnormality, leakage current may flow through the any one of the Zener diodes. The voltage of the single cell connected in parallel with the failed Zener diode decreases due to the leakage current, so it is required to detect the abnormality of the Zener diode. In Japanese Patent Application Publication No. 2010-140785 (JP 2010-140785 A), an additional failure detection switching element is provided in a path for charging or discharging a battery in order to detect a failure of an open/close unit (switching element). A failure of the open/close unit is detected by detecting an on/off state of the open/close unit while the failure detection switching element is subjected to on/off control.
In JP 2010-140785 A, another failure detection switching element that is able to undergo on/off control needs to be provided in order to detect a failure. Therefore, a circuit configuration becomes complex, and a failure detection target is limited to the switching element. For example, it is not possible to detect a failure of an element that is used in an overvoltage protection circuit, such as a Zener diode.